Summary

September 2020 Weather Summary

September was warmer than normal. On the 4th, record highs of 102 were set at Boise and Burns. The 99 at Ontario tied the record for the date. On the 5th, the high of 100 at Boise tied the record for the date, and the 98 at Twin Falls set a new record.
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It would have been even warmer had it not been for widespread smoke and haze from wildfires.


It was a dry month, especially in southcentral and southeast Oregon, where totals were less than 5 percent of normal across a wide area.
September precipitation departure
September temperature anomaly
A very warm upper-level high pressure ridge anchored over the western region kept temperatures above normal during the first week of the month. By the 8th the ridge had migrated far enough west to allow a very cool upper-level low pressure trough to drop south from Canada.


The cold front ahead of the trough crossed our area on the 7th. Following the front, northwest winds in the 40-50 mph range were common. A gust of 54 mph from the northwest was measured near Boise.
https://twitter.com/NWSBoise/status/1303333613392388097?s=20
The trough kept temperatures below normal. Highs failed to reach 70 at a number of locations on the 8th.
Following the trough, the ridge built inland, and temperatures were above normal again on the 11th. The ridge dominated through the 17th, with highs in the 80s at many lower valley locations.
The northern part of the ridge gave way to a Pacific cold front on the 18th. Most locations received only light precipitation. Isolated thunderstorms generated outflow winds over 40 mph.
From the 20th through the 25th, the ridge was alive and well over the southwest U.S., while southwest flow aloft on its northern periphery kept temperatures in our region above normal.
Meanwhile, a more fall-like pattern was developing as westerly flow aloft strengthened across the North Pacific.
The upper-level winds carried a fast-moving weather disturbance inland on the 25th. While rainfall was generally light in the valleys, McCall set a record for the date with .54 inch.


Temperatures stayed below normal through the 27th. On the 28th the ridge, now resurrected over the west coast, brought a warming trend. It would guarantee above-normal temperatures through the 30th and beyond.

August 2020 Climate

August was warmer than normal across the area, and a number of record highs were set.
August 2020 temperature (2)
August 2020 precipitation

On the 15th, the high of 99 at Burns set a daily record.

On the 16th, the high of 103 at Boise set a daily record. A record of 100 was set at Burns.

On the 17th, the high of 102 set a record for the date at Boise. Other record highs include 106 at Mountain Home and 99 at Baker City.

On the 18th, the 98 at Baker City tied the daily record.

On the 19th, the 97 at Twin Falls was a record for the date.

It was drier than normal nearly everywhere, but, as mentioned in the July summary, that’s actually normal.

Monsoon thunderstorms brought more wind than rain.

On the 15th, a thunderstorm near Andrews, Oregon broke off dead tree limbs and did other minor damage. At Little McCoy Creek in Harney County, a gust of 63 mph was measured by automated equipment.

On the 17th, a gust of 70 mph was measured by a spotter. A spotter in Fields, Oregon measured a sustained speed of 44 mph with a gust of 58 mph. A spotter in Fields measured a 54 mph gust which resulted in lost power. There were numerous other reports of gusts exceeding 50 mph.

On the 18th a spotter in Oreana reported half-inch diameter hail.

As usual, the seasonal upper level high pressure ridge over the southwest states was the main source of the heat. An occasional Pacific cold front provided some respite, but there was no significant cooling Until the end of the month.

Had the air been clear, it would have been even hotter. But widespread smoke from wildfires limited high temperatures on many days. Persistent southwest flow aloft transported the smoke from its California source.

The monsoonal moisture brought high dewpoints to the region, which helped fuel afternoon and evening thunderstorms from August 24th through the 29th. Boise recorded the highest dewpoint since September 5th, 2013 on August 25th.

On August 26th, a long-lived supercell thunderstorm tracked across Owyhee County with 2″ diameter hail.

Overnight on the 29th, a dry cold front brought much cooler air from western Canada. Highs were around 15 degrees lower than on the previous day.

Northwest winds at the surface and aloft behind the front temporarily swept away most of the smoke.

On the 31st, a weather system from British Columbia brought clouds, a breezy afternoon, and slightly cooler air.

July 2020 Climate

The temperature, when averaged across the area, was slightly above normal. The greatest departures were at Baker City (+3.5 degrees), McCall (+2.5 degrees), and Rome (+2.1 degrees).
July 2020 temperature
Precipitation was below normal nearly everywhere. July is one of the two driest months of the year (the other being August), so dry is actually normal.
July 2020 precipitation
During most of the month, upper-level low pressure troughs moving inland over western Canada would play a role in our weather, as they vied for dominance with the very warm seasonal upper-level high pressure ridge over the southwest U.S.

Precipitation is usually associated with troughs, but during July they produced little if any rain south of the Canadian border, bringing only brief periods of slightly cooler weather.

Between troughs, the high pressure ridge to our south made its presence felt as it expanded northward.

Increasing amounts of monsoon moisture circulating within the ridge eventually managed to reach our area on the 22nd and 23rd, and again on the 27th and 28th, carried north ahead of weak low pressure troughs over California.

On the 23rd, thunderstorms brought locally strong gusty winds. At Rome, numerous tree branches were blown down and garbage cans were knocked over. A gust of 64 mph was measured 21 miles north of Murphy Hot Springs. Numerous other locations reported gusts in excess of 50 mph.

On the evening of the 28th, thunderstorms crossed the Owyhee Mountains, bringing spotty but locally heavy showers to the Boise area.

The ridge made a major northward expansion on the 30th and 31st, bringing the hottest weather of the summer.

At Baker City on the 30th, the high temperature of 101 tied the daily record set in 2003. And the 105 on the 31st set a new record for the date.

On the 31st at Burns, the high of 99 tied the record set in 1949. At Ontario, the high of 107 tied the record set in 1971.


Pocatello tied their all time record high temperature of 104 degrees as well.

 

 

June 2020 Climate

Temperatures varied widely during the month. Most of southwest Idaho was cooler than normal. Most of southeast Oregon had near or slightly below normal temperatures, but Baker City, Burns, and Rome averaged warmer than normal.

From the 1st through the 4th, temperatures were above normal under dry and relatively warm westerly flow aloft. On the 5th, southwesterly flow ahead of a Pacific cold front brought even warmer air.

Gusty west winds followed the cold front. Many locations reported gusts in the 50-70 mph range. The strongest was 77 mph at an automatic observing site 10 miles southwest of Hill City.

https://twitter.com/NWSBoise/status/1269481164642738178

A cold upper level trough from the Gulf of Alaska followed the front. High temperatures were as much as 30 degrees cooler from the 6th through the 8th. Tamarack Ski recorded 17″ of snow, while 15″ fell at Brundage and 8″ at Bogus Basin.

Except for northern Harney County, and a narrow strip between the Snake River and the Owyhee Mountains, June was wet, especially in a corridor from southern Malheur County across the central Idaho mountains. The Treasure Valley was the wettest area in the region. Boise had the third wettest June on record.

On the 7th, rainfall set new records for the date at McCall (1.13 inches), Ontario (.35 inch), and Twin Falls (.28 inch). During the three day period from the 6th through the 8th, McCall measured 1.75 inches and Boise 1.11 inches of rain. Much less fell elsewhere. Baker City and Burns received only traces.

Following the trough, a high pressure ridge brought dry and warmer weather from the 9th through the 12th. The high of 90 at Twin Falls on the 12th set a record for the date.

The cold front ahead of the next trough arrived on the the 12th. Following the front, west winds gusted over 50 mph at many locations. The strongest reported gust was 61 mph at Grassy Mountain 19 miles southeast of Rome.

On the 13th, an upper level low pressure trough lifted north from California, generating a band of heavy rain which was mainly confined to the Treasure Valley. Many reporting points in Ada, Baker, and Washington Counties measured just over one inch. Record rainfalls for the date were set at Boise (1.21 inches) and Ontario (.74 inch).

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Rainfall from the 13th through the 17th totaled 1.81 inches at Boise, 1.11 inches at McCall, .83 inches at Ontario, and 1.31 inches at Rome. On the 13th, rainfall amounts set records for the date at Boise (1.21 inches) and Ontario (.74 inch). Outside of the heavy rain band, other locations in southeast Oregon received between a quarter and a half inch. Southeast of Boise, Mountain Home got only .11 inch and Jerome .12 inch.
On the 16th, yet another trough arrived from the Gulf of Alaska, keeping temperatures unseasonably cool through the 18th.

On the 24th, thunderstorms brought hail and strong gusts to a few locations. Four miles south of Jamieson in Malheur County, hail one inch in diameter covered the ground and caused damage to crops. Quarter inch hail fell nine miles north of Vale. Thunderstorm winds exceeded 50 mph at a few locations in Owyhee County.

On the 25th, the high of 94 at Twin Falls tied the record for the date set in 2015.

On the 27th another unseasonably cold low pressure trough was bearing down from the north.

The cold front ahead of this trough crossed our area late on the 27th. The air behind the front was much cooler and initially dry. but as the trough settled over the northern Intermountain Region, it entrained a plume of moisture from across the Pacific, resulting in moderate amounts of rain at several locations on the 29th and 30th. The most reported was .77 inch at Ontario.

At Twin Falls on the 28th, a daily rainfall record was set (.25 inch), and a record low temperature was set (42).

On the 29th, daily rainfall records were set at Ontario (.74 inch) and Twin Falls (.18 inch).

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Also on the 29th, the low of 41 at Twin Falls set a new record low for the date.

 


June 2020 temperatureJune 2020 precipitation

May 2020 Climate Statistics

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Temperatures averaged just over one degree above normal and precipitation was 40 percent above normal at Boise.

An extensive upper-level low pressure system over the northeast Pacific was the dominant feature through the 17th. It was responsible for steering weather systems into western North America. Between systems, southwest flow east of the low brought warmer air.

The first couple of weather systems, which came through on the 3rd and the 6th, brought only light precipitation. Strong west winds followed a cold front on the morning of the 6th. A gust of 48 mph was measured at the Boise Airport.

The first significant precipitation of the month came on the 14th when 0.36 inch of rain fell at the airport as a low pressure trough crossed the area.

On the 16th a much larger and colder trough from Siberia was nearing the west coast. Before moving inland on the 19th, it had deepened far enough down the coast to tap subtropical moisture southwest of California. On the 20th it was centered directly over Idaho. That day 0.80 inch of rain fell at the Boise Airport, breaking the previous record for the date of 0.56 inch in 1985. Other parts of town were drenched by even heavier rain, including 1.31 inches at Boise State University.

With a high of only 51 degrees, the 20th was the coolest day of the month. The record coolest high temperature for the date is 48 in 1902. Below-normal temperatures would continue through the 24th.

An upper-level high pressure ridge building over the coast on the 24th started a warming trend which was briefly interrupted by a weak Pacific cold front on the 26th.

Starting on the 27th, temperatures rose each day as the ridge strengthened. From the 28th through the 30th, highs were in the 90s for the first time since September. On the 30th the high of 98 tied the record for the date set in 1986. And the low of 69 broke the previous record of 65 set in 2015.

A low pressure system that had been spinning over the subtropical Pacific west of Baja finally moved inland over California on the 30th. As it progressed northward toward eastern Oregon and southwest Idaho, it was preceded by hot dry windy weather and scattered thunderstorms. Precipitation was mostly light. Only a trace of rain dampened the airport, but a few spots closer to the foothills got heavy downpours.

The peak gust at the airport was 48 mph from the east-southeast at 5:43 pm MDT. Winds were likely stronger elsewhere around the valley, as there were reports of trees damaged or blown over and power outages. There was a lot of tree damage across the region, especially in the mountains with this system where wind gusts up to 74 mph were reported. There were also some great photos of the damaged trees:

By that evening the first round of showers had moved north and east of Boise, and at 9:15 pm MDT a cold front passed the Boise Airport from the west.

There was more weather to come later that night, as the upper-level trough behind the cold front was still on its way. The arrival of the trough was announced by a thunderstorm just before 3 am MDT on the 31st. Hail one-half inch in diameter fell at the airport, but there was only 0.09 inch of total precipitation.

Thunderstorms between midnight and 6:00 am are very rare in our part of the country. At Boise the chance is only 2 percent in May and 4 percent in June.

Here are the rest of the climate graphics for southwest Idaho and southeast Oregon.

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April 2020 Climate Statistics

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Precipitation was near normal, and temperatures averaged slightly warmer than normal.

A cold upper level low pressure trough over western Canada and the Northwest states during the first three days of the month held temperatures below normal.

Southwest flow aloft ahead of a low pressure trough off the Pacific Northwest coast started a warming trend on the 4th.  As the trough moved south along the California coast on the 5th and 6th, showers spread inland, but the Boise area received only light precipitation.  The air was unstable enough for thunderstorms both days.

With our area between the low pressure system over the southwest states and the jet stream to our north and east, weak high pressure kept temperatures above normal from the 5th through the 11th.

A cool down was on the way as the flow over western Canada shifted into the north on the east flank of a high pressure ridge off the west coast of North America.

A cold front from British Columbia passed Boise during the evening of the 11th, accompanied by a wind gust of 45 mph from the north.  Temperatures the following day were 10 to 15 degrees cooler, and temperatures stayed below normal through the 14th.

Boise fared better than eastern Idaho, and much better than states to the east, where a late season arctic outbreak brought snow and winter temperatures.

After a brief warmup on the 15th, another cold front from British Columbia crossed the Boise area that evening. Compared to the previous front, the cool down was minor.  A convergence zone in the Snake River Basin near Mountain Home created show showers on the morning of the 16th.

A warming trend began on the 17th as the high pressure ridge began to move inland from the coast.

Increasing westerly flow aloft gradually weakened the ridge, and on the 22nd and 23rd a disturbance embedded in the flow brought the largest amount of precipitation for the month. Most of it fell on the 23rd with scattered convective showers, some of which produced brief heavy rain and small hail.

On the 25th and 27th a couple of weak disturbances dropped most of their precipitation over north Idaho, with only light showers at Boise.

On the 28th a strong high pressure ridge developed over the western U.S. as it amplified and drifted east, it provided the season’s warmest weather so far.

On the 29th strong southwest flow aloft on the west flank of the ridge, down valley flow at the surface, and mostly sunny skies helped boost the temperature at Boise to 88 degrees, one degree shy of the record set in 1992. Severe thunderstorms containing 1″ hail were reported in Baker City.

On the 30th the southwest flow carried an energetic weather disturbance across southeast Oregon and southwest Idaho. Late that morning a line of thunderstorms formed over Harney County.  The storms intensified rapidly as they approached Idaho. They crossed the border at about 1:45 pm, and strong outflow ahead of the storms arrived at the Boise airport at 2:30 pm.  Five minutes later a gust of 61 mph was measured just ahead of the rain, which was heavy at times.  Golf-ball size hail (1.75″) was reported at Oreana, ID (40 miles southwest of Boise).

 

The winds were likely stronger in some parts of town, as there were reports of power outages, downed trees, and other damage. Large trees across the Treasure Valley toward the Boise Mountains and Magic Valley were knocked down as a result of this storm.  Numerous large trees were knocked down from Prairie to Pine to Featherville with this storm.

 

Here are the rest of the climate graphics for southwest Idaho and southeast Oregon.  McCall was cooler than average due to a deep snow pack on the ground within the valley, which kept their temperatures lower than other areas.

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Winter 2019-2020 in Review

Winter 2019-2020 was much warmer than normal, with the greatest departures in the western portion of our County Warning Area (CWA). The average temperature for December through February at Ontario, Oregon was 7.4 degrees above normal, making it their third warmest winter on record. 

Snowfall was generally below normal, mainly because of the unusually warm temperatures, but below-normal precipitation in some areas also played a part.

Most of our forecast area had near normal precipitation for the season. Camas, Twin Falls, Gooding, and Jerome Counties, and the Owyhee Mountains, were drier than normal. Southern portions of Harney, Malheur, Owyhee, and Ada Counties were wetter than normal, as was Baker County, with the notable exception of the Baker Valley, which was significantly drier than normal.

One of the large-scale controlling mechanisms for winter weather in the Northern Hemisphere is the Arctic Oscillation (AO). It is characterized by a counterclockwise circulation of winds around the Arctic, centered at about 55 degrees north latitude. This winter it was in its positive phase, meaning that the winds were strong and consolidated.  This tends to hinder arctic air from moving south into the lower 48 states.  Most of this winter the AO was strongly positive, especially in January and February.  In fact it set all-time records twice in February.  

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New Daily Records

Dec 21 – McCall – record high temperature of 43 (old record 42 in 1906)
Dec 22 – McCall – record high temperature of 48 (old record 46 in 1940)
Jan 26 – Boise – record rainfall of .74 inch (old record .40 inch in 1970)
Feb 5 – Boise – record snowfall 2.6 inches (old record 2.1 inches in 1948)
Feb 5 – Baker City – record rainfall of .25 inch (old record .16 inch in 2010)

December

Temperatures were relatively mild, especially in the Treasure Valley. Ontario had its sixth warmest December on record. Baker City had its eleventh warmest. 

Aside from portions of Harney County, most of our CWA was drier than normal. Factors contributing to the precipitation deficit included a split in the jet stream, leaving our area between northern and southern branches. Most of the Pacific moisture moved south over the southwest states instead of east over our area. During periods of west or southwest flow over our CWA, some valleys were in the precipitation shadows of mountain ranges. This effect was especially pronounced at Baker City and Mountain Home, downwind from the Elkhorn and Owyhee Mountains. However,  precipitation at McCall was enhanced by lifting over the higher terrain to their east. 

Despite widespread below-normal precipitation, there were several storms which produced significant snowfalls. 

On the 1st and 2nd, 5 to 10 inches of snow fell in southeast Oregon and the mountains of southwest Idaho. Amounts were lighter in the Treasure and Magic Valleys, where only 1 to 3 inch amounts were reported.  

On the 8th, heavy snow fell in the mountains of Baker County Oregon and the west central Idaho and Boise Mountains. The greatest reported amount was 13 inches at Atlanta Summit and Deadwood. 

On the 12th, Bogus Basin received a 9-inch snowfall. Tamarack ski area got 14 inches, and Brundage accumulated 12 inches. The heaviest amount was 15 inches at Mores Creek Summit. 

From the 13th through the 16th there were relatively light amounts of snow across most of the area. Exceptions included 7 inches near Frenchglen in Harney County, 12 inches east of Three Creek in southern Twin Falls County, and 8 inches near Murphy Hot Springs in Owyhee County, all on the 14th. 

On the 19th, 6 inches was measured at Council in Adams County and at Placerville in Boise County, while 9 inches fell at Bear Saddle Snotel in Washington County.

On the 20th, 8 inches fell east of Meadows in Adams County and at Halfway in Baker County. Tamarack in Valley County picked up 11 inches. 

December 2019 precipitation

December 2019 temperature

January

Temperatures were above normal across nearly all of our CWA, and much above normal in the Treasure and Magic Valleys and southern Malheur County. Boise was +6.2° F above normal for the month – making it the fifth warmest January on record.

Strong westerly flow aloft maintained a progressive pattern which brought Pacific weather systems across our area at frequent intervals. This hindered the formation of long-lasting temperature inversions and discouraged arctic air from pushing south.

A persistent ridge of high pressure off the California coast steered storm systems, including a series of atmospheric rivers, into the Pacific Northwest. Atmospheric rivers are long narrow streams of air carrying large amounts of moisture. They are responsible for most of the water vapor transport outside the tropics. 

Most of our region was wetter or much wetter than normal. The mountains of central and northern Idaho and northeast Oregon saw a significant improvement in snowpack. 

Snow fell nearly every day in the mountains.

A storm on New Year’s Day dropped heavy snow on the mountains of Baker County and central Idaho. Storm totals included 14 inches at Tamarack and McCall, 15 inches at Brundage, and 5 inches at Bogus Basin. 

Strong winds were reported at many locations that day, including gusts of 67 mph  east of Van in northern Harney County, and 65 mph south of Rogerson in Twin Falls County. Elsewhere, gusts of 45 to 55 mph were common.

From the 9th and 14th, snowfall was heavy in the mountains. Even in the lower valleys moderate amounts fell. 

A storm total of 18 inches was reported at Centerville in Boise County.  In Valley County storm totals included 23 inches near Donnelly, 17 inches at Yellow Pine, and 9 inches at Cascade. In Baker County Halfway’s total was 17 inches. Featherville in Elmore County accumulated a total of 36 inches. 

Total snowfall at Boise for the six days was 8 inches, more than half the total this winter. The most on the ground was 4 inches on the morning of the 15th. 

A couple of places reported strong winds on the 14th, including a gust of 63 mph at Rome in Malheur County and 58 mph at Twin Falls.

January 2020 precipitation

January 2020 temperature

February

February was colder than January at several locations. Normally  February is around 5 degrees warmer than January. Still, Februrary was warmer than normal across most of our area. 

On the 5th and 6th a warm front, which had entrained tropical moisture from east of Hawaii, brought heavy snow to the mounains and moderate amounts of rain and snow to the valleys. 

In Baker County, 7 to 8 inches of snow fell near Baker City, and 12 inches was measured at Sumpter.  In Valley County 17 inches fell near McCall and at Yellow Pine. Near Donelly 9 inches was reported. In Adams County New Meadows got over 25 inches. The storm total at Brundage was 40 inches. 

A storm on the 16th dropped 4 to 5 inches of snow on Boise County. McCall received only 3 inches. Rain fell in the lower valleys. By far the greatest reported amount was 0.53 inch at Boise, which set a new record for the date.

During the afternoon and evening of the 23rd, gusty northwest winds followed a strong cold front. Some the the stronger gusts reported included 67 mph at Baker City, 77 mph at Wagontire in Harney County, 67 mph at Midvale Hill in Washington County, 53 mph at Burns in Harney County, 56 mph at Black Canyon in Payette County, and 65 mph near Hammett in Elmore County.

February 2020 precipitation

February 2020 temperature

February 2020 Climate Statistics

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February was colder than January for the third consecutive year. Normally December is the coldest month, with January half a degree warmer, and February about 5 degrees warmer than January.

With an average temperature of 36.9, winter (Dec-Feb) 2019-20 was the fifth warmest at the airport (80 years of records) and the eleventh warmest at all official observing locations in Boise (156 year of records). The season’s low was 16 on October 30 and January 15. The only other time the coldest “Winter” temperature occurred in October was in 2002, when the low was 13 on the 31st.

Snowfall for the season so far has been 15.4 inches, compared to the normal of 17.6 inches for October through February.

On the 1st an upper-level high pressure ridge kept temperatures above normal. A cold front passed Boise just after midnight that night. Northwest winds gusted to over 30 mph just behind the front. The strongest gust was 38 mph at 10:26 am MST on the 3rd. An upper-level low pressure trough, followed by northerly flow aloft, kept temperatures below normal from the 2nd through the 5th.

On the 5th and 6th a warm front, which had entrained tropical moisture from east of Hawaii, brought nearly half an inch of precipitation over the two-day period, including nearly 3 inches of snow ahead of the front on the 5th.

The central Idaho and northeast mountains had 36+” of snow with this system.

Following the warm front, temperatures stayed above normal starting on the 6th under the influence of an upper-level high pressure ridge parked off the west coast. The ridge moved inland on the 12th and 13th, followed by fast and relatively mild westerly flow aloft on the 14th and 15th.

On the 16th a very moist disturbance embedded in the westerly flow brought a record amount of rain, most of which fell between 12:30 am and 12:30 pm MST. A cold front crossed the Boise area at about 12:45 pm MST, ending the main rain event. Unstable air behind the front set off a brief thunderstorm with small hail around 3:30 pm MST. The total precipitation of.53 inch set a new record for the date, exceeding the previous record of .40 inch in 1976.

Temperatures stayed below normal from the 17th through the 21st due to northwest flow aloft and cool surface high pressure.

A brief warmup followed on the 22nd and 23rd ahead of a strong cold front. The front crossed the Boise area at 6:19 pm MST on the 23rd with only a trace of rain. The main impact was strong north-northwest wind which gusted to 48 mph at the airport at 6:46 pm MST. Temperatures stayed below normal through the 25th.

Temperatures gradually warmed from the 26th through the 28th as an upper-level high pressure ridge crossed the intermountain region. The high for the month was 63 on the 28th.

A dry cold front moved through early on the morning of the 29th, and the high that day was 15 degrees cooler.  Here are the climate graphics for selected cities across southeast Oregon and southwest Idaho.

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January 2020 Climate Statistics

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It was the fifth warmest January at the airport, where records go back to 1940, and the eighth warmest January in the Boise area in 156 years of temperature records.

A strong westerly circulation maintained a progressive pattern which brought pacific weather systems across our area at frequent intervals. This hindered the formation of long-lasting temperature inversions. It also prevented arctic air from pushing as far south and west as Boise.

The low for the month, and so far this winter, was 16 on the 15th and also on the 30th of October. The probability is slightly above 50 percent that the temperature will fall below 16 in February.

Precipitation was nearly one inch above normal.

Snowfall totaled 8.9 inches, compared to the normal 5.1 inches. The temperature rose above freezing every day, limiting accumulation. The longest period of snow cover (1 inch or more) was five days from the 13th through the 17th. The greatest depth was 4 inches on the 15th.  Here are other stations across southwest Idaho and southeast Oregon.

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December 2019 Climate Statistics

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It was one of the milder Decembers. Only eight Decembers were warmer in the 80 years of airport records.  No temperature records were broken or tied.

The temperature rose above freezing every day except the 29th, when the high at the airport was 32 degrees.  However, nearby Boise locations warmed into the mid-30s that day.  Normally there are 7 days in December when highs fail to exceed 32 degrees.

The relatively warm weather was due the absence of long lasting temperature inversions, and a pattern which kept arctic air east of The Rockies.

The 22nd was the warmest day with 61 degrees, under strong southwest flow aloft ahead of an upper level trough.  It was also the windiest day, with a gust of 36 miles per hour from the southeast.

Precipitation was half an inch below normal.  Snowfall was less than half normal.  The greatest snow depth was 1 inch on the 2nd.  The average December has 7 days with an inch or more on the ground.

The precipitation deficit was due to a tendency for storm systems to move south over California, rather than moving east over our area.

Nearly half of December’s precipitation fell from the 11th through the 14th, when a brief pattern change allowed moist westerly flow aloft to push inland over the northern intermountain region.

Across southwest Idaho and southeast Oregon, it was a common theme of above normal temperatures and below normal precipitation and snow.

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